Electron beam devices



2,992,355 ELECTRON BEAM DEVICES Joseph Feinstein, Livingston, 'N.J.,assignor to Bell Telephone Laboratories, lncorp'orated, New York, N.Y.,a corporation of New York Filed Oct. 26, 1959, Ser. No. 848,831 19Claims. (Cl. 31'53.5)

This invention relates to electron beam devices and more particularly tothe reduction of noise in the beam of such devices.

Electron beam devices such as traveling wave tubes have proven capableof microwave amplification with reasonably high gain and stability overan exceedingly wide band of frequencies. Detracting from the significantadvantages realized by such devices, however, is the noise resultingfrom the utilization of an electron beam. Considerable research on thenature of electron beams has taken place in an effort to reduce thisundesirable noise as much as possible. In a paper entitled The MinimumNoise Figure of Microwave Beam Amplifiers, by H. A. Hans and F. N. H.Robinson, Proceedings of The Institute of Radio Engineers, volume 43, p.981-991, August 1955, equations are developed that lead to a theoreticalminimum noise figure of a conventional traveling wave tube ofapproximately six decibels.

Subsequent to the Work of Hans and Robinson, workers in the artdiscovered that certain particular electron guns in conventionaltraveling wave tubes sometimes yielded noise figures of less than sixdecibels. Investigation has indicated that these apparent contradictionsof theory result from the fact that the Hans-Robinson calculations arebased on the assumption that velocity variations in any given transverseplane of the electron beam are negligible by comparison to the D.-C.beam velocity. This assumption is apparently valid in regions of highD.-C. beam velocity, but is not valid in the region adjacent the cathodewhere the beam has not been accelerated. In the cathode region, whereelectron velocity variations are large by comparison to the D.-C. beamvelocity, certain beam noise parameters, which were priorly consideredinvariant, have been found to fluctuate. After the beam has beenaccelerated so that these velocity variations are negligible, the noiseparameters no longer fluctuate. It has therefore been theorized thatcertain electron guns yield unexpectedly low noise figures because theelectron beam is suddenly accelerated at a point at which certain of thefluctuating beam noise parameters are at a null.

Because of the complex nature of an electron beam in the cathode region,a comprehensive understanding of the effects of velocity variationstherein has not been attained. It is known, however, that the resultingnoise parameter fluctuations are very rapid and therefore the distancebetween any given maximum and null of this fluctuation is extremelysmall. Practical application of this phenomenon has therefore beenhindered for two primary reasons: the exact location of a noiseparameter null is not readily predictable; the physical dimensions ofapparatus for accelerating the beam at some noise parameter null must beaccurate to prohibitively exacting tolerances.

It is an object of this invention to achieve low noise operation of anelectron beam device.

It is another object of this invention to take full advantage of thephenomenon of fluctuating noise parameters in a multivelocity electronbeam to produce low noise amplification.

These and other objects of this invention are attained in anillustrative embodiment thereof comprising an electron beam devicehaving a cathode for forming and projecting an electron beam and aninteraction region Patented July 11, 1961 wherein the beam is allowed tointeract with an electromagnetic wave. Between the cathode and theinteraction region are a series of electrodes which serve to minimizebeam noise as will be explained hereinafter.

In the multivelocity cathode region of the electron beam, the nature ofthe beam is so complex that reliable, sophisticated apparatus has not,to my knowledge, been devised to take full advantage of the fluctuatingnoise parameters. According to an aspect of my invention, I thereforeinclude in a traveling wave tube means for introducing controlledtransverse velocity variations in the electron beam. These controlledvelocity variations are introduced in a relatively high beam velocityregion so that the inherent beam velocity variations are negligible.Because the velocity variations are artificially introduced, and becausethe mean, or D.-C., beam velocity is appropriately high, the variousbeam parameters are determinable, and it is possible to predict thepoint at which the beam noise parameter is at a null. Further, the beamnoise parameter fluctuations are then relatively slow so that beamacceleration can be effected with fairly simple apparatus.

It is a feature of this invention that there be included between thecathode and interaction region of a beam device a pair of electrodes forintroducing transverse velocity variations on the beam. These twoelectrodes have slightly different voltages thereon so that electronsadjacent the more positively charged electrode will be given a slightlyhigher velocity than those which are nearer the other electrode.

As will be explained hereinafter, I have found that the noise figure ofa multivelocity beam fluctuates at a rate proportional to the beamsplasma frequency. More specifically, the beam noise figure reaches nullsat distances approximately equal to n/ 4 times the plasma wavelength,where n is some odd number. Accordingly, it is another feature of myinvention that the pair of electrodes used for introducing transversevelocity variations be of a length equal to 11/4 times the plasmawavelength of the beam, where n is some odd integral number.

The minimum noise figure represented by the aforementioned null will bemaintained throughout the interaction region of the tube only if thebeam is suddenly accelerated to a high D.-C. velocity. Accordingly, itis another feature of this invention that a velocity jump transducer beincluded immediately adjacent the downstream end of the aforementionedpair of conductive electrodes.

A more complete understanding of these and other features of the presentinvention can be gained from a consideration of the following detaileddescription, taken in conjunction with the attached drawing, in which:

FIG. 1 is a schematic view of a traveling wave tube embodying theconcepts of my invention;

FIG. 1a is a graph of the potential of the electron beam of the deviceof FIG. 1 versus distance;

FIG. 1b is a graph of the magnitude of the noise parameter of theelectron beam of the device of FIG. 1 versus distance; and

FIG. 2 is a schematic view of an electron gun illustrating anotherembodiment of my invention.

Referring now to FIG. 1, there is shown a traveling wave tube 10including an evacuated envelope 11 which encompasses a cathode 13 and acollector 14. Cathode 13 produces an electron beam which is focused bybeam forming electrode 16 and projected toward the collector byaccelerating electrode 17.

In accordance with my invention there is also included a pair ofparallel plate electrodes 20 and 21 bordered by an acceleratingelectrode 22, the purpose of which will be fully explained hereinafter.A D.-C. source such as a battery 25 is connected as shown to maintainthe various electrodes at suitable potentials with respect to eachother.

A focusing means, such as an electromagnet 26, is used to constrain thebeam through the production of a magnetic field B which is parallel withthe direction of beam flow, as is well known in the art.

Also included within envelope 11 is a conductive helix 27. Highfrequency electromagnetic wave energy is applied to the helix as shownby the input arrow. Thereafter, it is amplified in a known manner byinteraction of the resulting field, traveling on the helix, with thebeam. After amplification, the electromagnetic wave energy is extractedfrom the helix as shown by the output arrow. It is to be understood thatthe helix is merely exemplary of many various types of slow wavestructures which can be used.

In an electron beam device such as traveling wave tube 10, one of themajor obstacles in attaining optimum operation is the noise, or spuriouscurrent and velocity fluctuations, present on the beam. Noise is usuallyof a random nature and has two principal sources. One is thermalagitation in the input circuit and the other is irregularities in theemission of electrons from the cathode, the latter effect being known asshot noise. The noise figure of an electron beam device is the ratio,usually expressed in decibels, of the total noise output power to thenoise output power attributable to thermal noise at the input.

The investigations by Haus and Robinson, as disclosed in theaforementioned paper, led to the following relationship:

Where F is the theoretical minimum noise figure in a beam device, k isBotzmanns constant, T is a reference temperature, taken as 293 degreesK., and S and II are noise parameters which are defined in the paper.This relationship is well known to workers in the art. Equally wellknown is the fact that in the high velocity regions of a conventionalbeam device the noise parameter (SII) is invariant and the theoreticalminimum noise figure is approximately six decibels.

Subsequent to the Haus-Robinson paper, it was determined that in the lowvelocity region of beam, that is, the region adjacent the cathode, thenoise parameter (S--II) is not invariant but actually fluctuates withdistance to an appreciable degree. Calculations of noise fluctuationsalong a multivelocity beam were made by the densityfunction method ofanalysis and presented in the paper entitled Density-FunctionCalculations of Noise Propagation on an Accelerated MultivelocityElectron Beam, by A. E. Siegman, D. A. Watkins, and Hsung-Cheng Hsieh,Journal of Applied Physics, volume 28, No. 10, pp. ll381 148, October1957. These calculations were made with the aid of a computer and it wasfound that the noise figure at certain distances along the beam was aslow as three and one-half decibels. It was also shown mathematicallythat if the beam is abruptly accelerated at a point at which the noiseparameter (S-II) is at a null, the resulting low noise figure will bemaintained.

Although the analysis of Siegman et al. provides a better understandingof the nature of an electron beam, it does not prescribe a practicalmethod for taking advantage of the multivelocity beam characteristics inthe cathode region. The main difficulty in making use of these findingsis the extremely rapid fluctuations of the noise parameter (S-1I). Inphysical terms, one might say that the drift distance in which thecurrent fluctuations change to velocity fluctuations is extremely short.No beam accelerating apparatus has been devised which can reliablyaccelerate the beam at the precise point along the multivelocity regionadjacent the cathode where the noise parameter is at a predicted null.Further, since the noise parameter fluctuation is so rapid, the physicaldimensions involved would have to be accurate to the most exactingtolerances and the necessary velocity jump would have to be exceedinglyabrupt.

In my study of multivelocity beams, I have used a different analysisthan that hereinbefore described. Rather than the distribution functionapproach, I have studied the beam from the standpoint of wavepropagation. My analysis is rather complex and lengthy and so, for thesake of brevity, will not be herein included. Suflice it to say that mycalculations show that the noise parameter (S1I) fluctuatesquasiperiodically with distance as a function of the plasma wavelengthin a constant velocity region of the beam.

The beam parameter referred to as plasma wavelength is related to aphenomenon known as plasma oscillation. When a beam is modulated by asmall quantity of power, individual electrons which are displaced fromtheir equilibrium position tend to return to that position. Since theelectron has inertia, it will be carried to a point on the other side ofequilibrium where the momentum is overcome by opposing restoring forces.The resulting oscillation is similar to that of a weight supportedbetween two opposed compression springs. The product of the charge on anoscillating electron and the charge density of the surrounding space canbe considered as being analogous to the stiffness or elasticity of thespring. Since the charge-to-mass ratio is the same for all electrons,the plasma frequency is dependent on the charge density of the beam. Thedistance along the beam an electron will travel during one cycle ofplasma oscillation is the plasma wavelength. The plasma wavelentgh istherefore determined by the plasma frequency and the D.-C. velocity ofthe beam.

Because the parameter (SI[) varies as a function of the plasmawavelength, it is possible to adjust the frequency of variation byadjusting the plasma wavelength. This can be done by adjusting the D.-C.beam velocity. However, when the beam is accelerated to a velocity atwhich the plasma wavelength is long enough to permit the use of velocityjump apparatus of a practicable size, the beam is essentially of asingle velocity and the parameter (S-H) is substantially invariant. Itis therefore necessary to introduce artificial velocity variations sothat the beam will have the multivelocity characteristics previouslydiscussed.

Referring to FIG. la, curve 31 indicates the Variations of beampotential with distance along the tube 10. As shown by curve 31, thebeam potential falls slightly below the cathode potential at a smalldistance therefrom due to space charge concentrations. It then rises tothe potential V of accelerating electrode 17. The beam velocity can beconsidered as being also represented by curve 31 since beam velocity isproportional to the square root of beam potential. Curve 32 of FIG. lbindicates the variation of the noise parameter (S-H) with distancescorresponding to those of curve 31. The variation of (S1I) near thecathode results from the multivelocity characteristics of the beam atthat region. As the beam is accelerated, the inherent velocityvariations become negligible with respect to the D.-C. velocity and theparameter (SII) becomes invariant.

After being accelerated by electrode 17 the beam flows between parallelplate electrodes 20 and 21. As shown by the connections to battery 25,plate 20 is at a slightly higher potential than the potential V onelectrode 17, while plate 21 is at a slightly lower potential.Considering the deviation from the mean potential V produced by eachelectrode to be V electrodes 20 and 21 produce a potential distribution2V across the beam as shown by the dashed portion of curve 31. Theresulting transverse velocity variations are illustrated by arrows 34,the relative lengths of which are intended to illustrate relativemagnitudes of electron velocity. After passing between plates 20 and 21,the beam is abruptly accelerated by electrode 22 having a potential V asshown by curve 31.

From curve 32 one can see that the parameter (S-II) varies much moreslowly in the multivelocity region between electrodes 17 and 22 than inthe multivelocity region immediately adjacent the cathode. The reasonfor this is the high beam velocity and hence the longer plasmawavelength between electrodes 17 and 22. As

previously mentioned, the (8-11) fluctuates quasiper-- iodically as afunction of the plasma wavelength. More specifically, I have found thatminima in the (8-11) parameter reoccur at distances approximately equalto one-fourth of the plasma wavelength. The curve 3 2 is intended toillustrate a minimum at the position of accelerating electrode 22. Theplates 20 and 21 of the embodiment of FIG. 1 are therefore approximatelyonefourth of a plasma wavelength long. If plates 20 and 21 wereextended, my calculations show that other minima would occur atdistances approximately equal to n/4 times the plasma wavelength, wheren is any odd number. Since the noise parameter fluctuations are notexactly periodic, however, a longer electrical length of plates 20 and21 than that shown would probably result in a greater margin of error.However, because of the long plasma wavelength associated with the highpotential region of plates 20, 21, a length of one-fourth plasmawavelength is easily physically realizable.

In the course of my calculations, certain assumptions were made, amongthem the assumption that the potential deviation V be small with respectto the mean beam potential V On the other hand, I have found that thenoise figure is reduced as the potential deviation V is increased.Further investigation has indicated that a convenient ratio of potentialdeviation to mean potential where a is the ratio of plate separation tobeam thick- As previously pointed out, the velocity jump which the beamexperiences after drifting between plates 20 and 21 must be abrupt. Itcan be shown that the transition from the beam potential of V V to theaccelerated potential of V (see curve 3 1) must take place in a distancewhich is short compared to the plasma wavelength divided by 211'. Sinceplates 20 and 21 are only one-quarter of a plasma wavelength long, it isseen that this condition is easily realizable.

In FIG. 2 is shown a traveling Wave tube 36 which illustrates anotherembodiment of my invention utilizing a hollow beam. The various elementsof this embodiment function in the same way as the device of FIG. 1 andso have been numbered accordingly. Cathode 13 comprises an annular ringcoated with emissive material for producing a hollow electron beam as iswell known in the art. Electrodes 16, 17 and 22 are the same as thecorresponding electrodes of FIG. 1 except that they are constructed withannular apertures for the passage of an annular electron beam. Electrode20 is in the form of a hollow cylinder rather than a plate as shown inFIG. 1. Electrode 21 is a solid cylinder which is coaxial with electrode20. The voltages of the electrodes 17, 20, 21 and 22 are labelledaccording to the terms defined with reference to FIG. 1. A rod ofinsulating material 37 extends along the tube axis to support thecentral portions of the various electrodes.

The advantages of using a hollow beam in certain instances are wellknown. The method by which my invention is adapted to such a device isself-explanatory by the illustration of FIG. 2. As shown by the arrows34, the desired transverse velocity spread is produced around the entirecircumference of a portion of the hollow cylindrical beam. The graphs ofFIGS. la and 1b apply also to the device of FIG. 2.

It is to be understood that the above-described embodiments are onlyillustrative of the application of the principles of my invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An electron beam device comprising a cathode for forming an electronbeam, a collector for collecting said beam, means for constraining saidbeam to follow a single predetermined path of flow, a first acceleratingelectrode, a second accelerating electrode, two elongated electrodesparallel with said beam and between said first and second acceleratingelectrodes, means comprising a D.-C. voltage source for producing apotential difference between said parallel electrodes, and means forcausing electromagnetic wave energy to interact with said electron beam.

2. An electron discharge device comprising means for forming andprojecting an electron beam along a path, said means comprising acathode and a first accelerating electrode, said beam beingcharacterized by a plasma wavelength which is a function of the meanvelocity of said beam, means for causing an electromagnetic wave tointeract with said electron beam, a second accelerating electrodepositioned between said first accelerating electrode and said means forcausing interaction, a pair of parallel electrodes positioned betweensaid first and second electrodes and parallel with said electron beam,each of said parallel electrodes having a length approximately equal toone-fourth of said plasma wavelength in the region between said firstand second accelerating electrodes and means comprising a D.-C. voltagesource for producing a potential difference between said parallelelectrodes.

3. A device of the type which effects interaction of electromagneticwave energy with an electron beam comprising means for forming andprojecting a beam of electrons along a path, said beam beingcharacterized by a noise parameter which is determinative of thetheoretical minimum noise figure of said device, said noise parameterbeing invariant when the mean velocity of said beam is large withrespect to beam velocity variations, first means for accelerating saidbeam thereby making any inherent beam velocity variations small withrespect to the mean beam velocity, means downstream from said firstaccelerating means for producing artificial beam velocity variationsacross a transverse portion of said beam thereby producing fluctuationsof said noise parameter, said last-mentioned means comprising a D.-C.voltage source second means for accelerating said beam positioneddownstream from said velocity variation producing means at a positionalong said beam path at which said noise parameter is at a minimum, andmeans for collecting said beam.

4. An amplifier comprising a substantially tubular evacuated envelopeenclosing a cathode for forming an electron beam and a collector forsaid electron beam, said cathode and collector being at opposite ends ofsaid envelope, means for producing a magnetic field parallel with theaxis of said envelope, a first electrode within said envelope beingmaintained at a higher positive D.-C. potential than said cathode, asecond electrode between said first electrode and said collector beingat a higher positive DC. potential than said first electrode, a pair ofelongated electrodes each parallel with said axis and extending alongthe same portion thereof, one of said elongated electrodes being at ahigher D.-C. potential than said first electrode and at a lower D.-C.potential than said second electrode, the other one of said elongatedelectrodes being at a lower D.-C. potential than said first electrodeand at a higher potential than said cathode, and means included betweensaid second electrode and said collector for causing electromagneticwave energy to interact with said electron beam.

5. The amplifier of claim 4 wherein the difference of potential betweenone of said elongated electrodes and said first electrode is the same asthe difference of potential between the other of said elongatedelectrodes and said first electrode.

6. An electron beam device comprising a cathode for forming an electronbeam, a collector for collecting said beam, means for constraining saidbeam to follow a single predetermined path of flow, a first acceleratingelectrode, a second accelerating electrode, a pair of conductive platesparallel with said beam and on opposite sides thereof, said pair ofconductive plates being between said first and second acceleratingelectrodes and at equal distances from said cathode, means for producinga D.-C. potential difference between said conductive plates, and meansfor causing electromagnetic wave energy to interact with said electronbeam.

7. The electron beam device of claim 6 wherein said electron beam ischaracterized by a plasma wavelength which is a function of the meanvelocity of said beam, and wherein each of said conductive plates has alength substantially equal to one-fourth of the plasma wavelength. ofthe beam in the region between said first and second electrodes.

8. An electron discharge device comprising means for forming a hollowelectron beam, a collector for collecting said beam, means forconstraining said beam to follow a single predetermined path of flow, afirst accelerating electrode, a second accelerating electrode, a pair ofcylindrical coaxial electrodes separated by an annular space forallowing passage therethrough of said hollow electron beam, said coaxialelectrodes being positioned between said first and second acceleratingelectrodes, means for producing a potential diiference between said pairof coaxial electrodes, and means for causing electromagnetic wave energyto interact with said electron beam.

9. An electron beam device comprising the following elements positionedalong an axis in the order recited: a cathode, a focusing electrode, afirst accelerating electrode, a pair of elongated electrodes producingtherebetween an electric field which is transverse to said axis, asecond accelerating electrode, a conductive helix, and a collector; andfurther comprising an envelope encompassing all of the aforesaidelements, means for maintaining said first accelerating electrode at ahigher D.-C. potential than said cathode, means for maintaining saidsecond accelerating electrode at a higher potential than said firstaccelerating electrode, and means for producing a focusing field alongsaid axis.

10. An electron discharge device having a central axis and comprisingmeans for forming and projecting a beam of electrons along said axis,said forming means comprising a cathode, said beam being characterizedby a plasma wavelength which is a function of the mean velocity of saidbeam, a collector, a first accelerating electrode, a second acceleratingelectrode positioned between said first accelerating electrode and saidcollector, and third and fourth electrodes positioned between said firstand second elec- 8 trodes, said third and fourth electrodes producingtherebetween an electric field which is transverse to said electronbeam.

11. The electron discharge device of claim 10 wherein said third andfourth electrodes are each one-fourth of the plasma wavelength of thebeam in the region between said first and second electrodes.

12. The electron discharge device of claim 11 wherein said third andfourth electrodes are parallel and equidistant from said secondelectrode, the distance between said third and fourth electrodes andsaid second electrode being small with respect to the plasma wavelengthof said beam in the region between said first and second electrodesdivided by 21r.

13. The electron discharge device of claim 12 wherein said firstelectrode is at a higher D.C. potential than said cathode, said secondelectrode is at a higher D.-C. potential than said first electrode, saidthird electrode is at a higher D.-C. potential than said first electrodeand at a lower D.-C. potential than said second electrode, and saidfourth electrode is at a lower D.-C. potential than said first electrodeand at a higher D.-C. potential than said cathode.

14. The electron discharge device of claim 13 wherein said third andfourth electrodes comprise a pair of parallel conductive plates.

15. The electron discharge device of claim 13 wherein said third andfourth electrodes comprise a pair of coaxial conductive cylinders.

16. An electron gun for producing a low noise electron beam comprising acathode, a first accelerating electrode for accelerating the electronbeam from said cathode to a mean beam velocity at which any inherentbeam velocity variations are small with respect to said mean beamvelocity, means downstream from said first accelerating electrode forproducing artificial beam velocity variations across a transverseportion of said beam, and a second accelerating electrode closelyadjacent said lastmentioned means and downstream therefrom for providingan abrupt increase in the mean velocity of the beam.

17. The electron gun of claim 16 wherein said beam is characterized by aplasma wavelength which is a function of its mean velocity, said beamvelocity variation producing means extending along said beam a distancesubstantially equal to 11/4 times the plasma wavelength of the beam atthe mean beam velocity produced by said first accelerating means, wheren is some odd integral number.

18. An electron discharge device comprising means for projecting anelectron beam, said projecting means including a cathode and a firstaccelerating electrode, means for collecting said beam, and means forreducing the noise in said beam, said last-mentioned means includingelectrode means positioned on opposite sides of said beam adjacent saidfirst accelerating electrode and downstream therefrom, means forapplying potentials to said electrode means to produce artificial beamvelocity variations across a transverse portion of said beam, andvelocity jump means including a second accelerating electrode directlyadjacent said electrode means and downstream therefrom for providing anabrupt increase in the mean velocity of said beam.

19. An electron gun for producing a low noise electron beam comprisingmeans for forming a beam of electrons which inherently includes aquantity of noise energy which fluctuates quasiperiodioally withdistance when substantial beam velocity variations exist over atransverse portion of the beam, means for accelerating said electronbeam from said beam forming means to a velocity at which any inherentbeam velocity variations are negligible with respect to the mean beamvelocity, means downstream from said accelerating means for producingartificial beam velocity variations over a transverse portion f Saidbeam thereby inducing said quantity of inherent References Cited in thefile of this patent UNITED STATES PATENTS Field Sept. 11, 1956 PeterOct. 16, 1956 Field et a1. July 23, 1957 Tien et al. I July 23, 1957Pierce Mar. 3, 1959

